The present invention relates to paperboard trays and more particularly to a paperboard tray having features which make it particularly suitable for use where a film liner is to be formed in or inserted into the tray in an automated manufacturing process.
Many products are merchandised in sealed cartons in high volumes but at a relatively low profit margin on a per carton basis. Examples of high volume, low margin products are frozen food products such as vegetable entrees or complete dinners which are packaged in closed cartons before freezing. Such products are maintained in a frozen state until reheated by a consumer in a conventional or microwave open.
The packaging operation by which the food products are sealed in individual cartons should be highly automated and should employ low cost materials to reduce the packaging costs without reducing the integrity of the package seal. Paperboard is strong and relatively inexpensive, and therefore highly suitable for many packaging applications.
U.S. Pat. No. 3,932,105, issued Jan. 13, 1976, and assigned to the assignee of the present invention, discloses equipment and a process for lining an erected paperboard tray with a continuous sheet of film. In a preferred embodiment, the film is heated and then drawn into intimate contact with the interior tray walls by applying a vacuum to the exterior bottom wall and exterior side walls of the tray.
One type of tray disclosed for use in the patented process includes rectangular flanges connected to the tops of the tray side walls and extending outwardly therefrom in a plane parallel to the plane of the bottom wall of the tray. This tray is erected by gluing tabs on the side wall panels against the exterior surfaces of the adjacent side walls and by gluing the flanges together where they overlap at the corners. A film liner is then formed in or inserted into the fully erected tray.
In a process disclosed and claimed in co-pending application Ser. No. 928,357, filed on July 27, 1978 Now U.S. Pat. No. 4,200,481 allowed Apr. 29, 1980 and assigned to the assignee of the present invention, a process is disclosed in which a paperboard tray at the start of a lining operation is partially erect; i.e., with side wall flaps glued against adjacent side walls but with flanges partially erect rather than bent horizontal. A mandrel flattens partially erect flanges against a support while also forcing an interposed film into contact with the flange surfaces. When the film is bonded to the flanges either due to its inherent adhesive properties by heat sealing or through the use of supplemental adhesives, the film tends to lock the flanges in place, and to rigidify the tray.
As indicated above, the tray disclosed for use in the patented process has overlapping flange panels. It was found that such flange panels create some risk that a tray will jam during feeding or will cause the flanges to be crumpled when the mandrel disclosed in the co-pending application is used to bend the flanges while bonding the overlying film. Moreover, at the point of overlap of the flanges, a double thickness discontinuity exists which might inhibit a tight peripheral seal when a lid or second film is placed over the tray.
Most paper materials tend to be stiffer in one direction than in another as a consequence of the manufacturing processes by which they are made. Paperboard can be more readily bent when the fold line is parallel to the grain of the material than when the fold line is transverse to the grain. Because the resistance to deformation of the paperboard is a function of the grain direction of the paper, the flanges of a tray subject to uniform flange-bending forces will deform to a lesser extent where the flange fold line is transverse to the grain than where it is parallel to the grain. In other words, adjacent flanges of a tray may end up in different planes, making it more difficult to produce a "perfect" tray in an automated manufacturing process.